MacPro migration to second hard drive on same machine

MACPro late 2012; start up disk running El Capitan. I have a second drive in this machine with a clean install of High Sierra. I would like to migrate my apps, files, etc. from the El Capitan disk to the High Sierra disk. I backed up with Time Machine. Here's the question: when I look at Migration Assistant I don't see any way to select the other disk with High Sierra. Am I doing something wrong? Suggestions?

Thanks in advance.